Recently, in portable electronic devices such as a portable telephone or an electronic information terminal, a secondary battery that can be discharged or charged, e.g., a lithium ion battery or a nickel hydride battery, is frequently used. The portable electronic device using the secondary battery includes a charging terminal in its body. The secondary battery is charged by connecting the terminal to a charger.
The portable electronic device is ordinarily held near a user to be always ready for use. Accordingly, when the charging terminal is exposed from the body of the portable electronic device, there is a possibility that the user will touch the charging terminal and receive an electric shock. There is also a possibility that metal will touch the charging terminal to cause short-circuiting of the portable electronic device. Recently, therefore, there has been offered a non-contact power transmission apparatus that can charge the portable electronic device in a non-contact manner without using any charging terminal.
As the non-contact power transmission apparatus, there is known a non-contact power transmission device of an electromagnetic induction type in which a charger includes a primary coil for power transmission and a portable electronic device includes a secondary coil for power reception. Power can be transmitted in non-contact by supplying current to the primary coil to generate a magnetic flux and causing the secondary coil to generate an induced electromotive force by the magnetic flux.
However, in the non-contact power transmission device of the electromagnetic induction type, when there is any metal component around the primary coil or the secondary coil, the magnetic flux generated at the primary coil may leak to the metal component. The leakage of the magnetic flux to the metal component creates problems of not only a drop in power transmission efficiency but also heat generation at the metal component.
Particularly, in a portable electronic device required to be miniaturized and made thin, a secondary coil and a metal component are arranged closer, and more magnetic fluxes easily leak due to the metal component. Thus, JP2000-201442A (Patent Literature 1) discloses a non-contact power transmission device that can prevent leakage of a magnetic flux to a metal component. FIG. 1 schematically shows the non-contact power transmission device disclosed in Patent Literature 1.
As shown in FIG. 1, the non-contact power transmission device includes primary coil 2 disposed in charger 1 and secondary coil 4 disposed in portable electronic device 3. Primary coil 2 and secondary coil 4 are arranged to face each other when portable electronic device 3 is mounted on charger 1. Primary coil 2 receives current to generate a magnetic flux Φ1. By supplying the current to primary coil 2 in the mounted state of portable electronic device 3 on charger 1, the magnitude of the magnetic flux Φ1 applied on secondary coil 4 is greatly changed. As a result, an induced electromotive force is generated at secondary coil 4, and power is transmitted from primary coil 2 to secondary coil 4 in non-contact.
The non-contact power transmission device disclosed in Patent Literature 1 further includes magnetic-substance layer 5 disposed to cover a surface opposite the surface of secondary coil 4 facing primary coil 2. Since magnetic flux Φ1 passes through magnetic-substance layer 5, leakage of magnetic flux Φ1 to the metal component around the non-contact power transmission device can be reduced. As a result, a drop in power transmission efficiency and heat generation at the metal component can be prevented.
However, when magnetic flux Φ1 of a size equal to or larger than an acceptable range is applied to magnetic-substance layer 5, magnetic saturation occurs at magnetic-substance layer 5. The magnetic saturation causes leakage of magnetic flux Φ1 from magnetic-substance layer 5 to the metal component disposed around metal-substance layer 5, creating a possibility that the power transmission efficiency will drop and heat will be generated at the metal component. Thus, in the non-contact power transmission device disclosed in Patent Literature 1, the magnitude of magnetic flux Φ1 is limited to prevent magnetic saturation at magnetic-substance layer 5.
A factor for determining the magnitude (hereinafter, “saturated magnetic flux”) of magnetic flux Φ1 causing magnetic saturation at magnetic-substance layer 5 is the thickness of magnetic-substance layer 5. The saturated magnetic flux at magnetic-substance layer 5 can be enlarged by forming magnetic-substance layer 5 thick in a direction perpendicular to that of the magnetic flux Φ1 passed through magnetic-substance layer 5. However, thicker magnetic-substance layer 5 leads to enlargement of the non-contact power transmission apparatus, causing enlargement of portable electronic device 3.
To prevent magnetic saturation at magnetic-substance layer 5, the current supplied to primary coil 2 can be reduced to increase the magnitude of magnetic flux Φ1. However, this causes a drop in amount (hereinafter, “power transmission efficiency”) of power transmitted per unit time. The drop in power transmission efficiency necessitates longer time for charging portable electronic device 3.